The present invention relates to power tools, and in particular to power tools having reciprocating drive mechanisms. Since jig saws are the most common type of power tool having a reciprocating drive mechanism, the present invention will be discussed in this connection. However, it is to be understood that the present invention has general applicability to any power tool that utilizes a reciprocating drive mechanism of the scotch yoke type.
Jig saws of the general type to which the present invention pertains typically comprise an electric motor which drives a reciprocating saw blade drive mechanism of the scotch yoke type. More particularly, the motor shaft is mechanically coupled to a crank plate having an eccentrically located crank pin that is drivingly disposed within the slot or "track" of a reciprocating yoke member. The yoke member is then generally coupled to the saw blade carrier. In all known prior art designs, the configuration of the yoke track is straight, perpendicular to the shaft of the yoke, with the length of the track equal to the diameter of the circle traced by the crank pin.
Although certainly functional, the disadvantage of the straight track design is that it creates a couple on the shaft of the yoke, the moment of which is greatest at the mid-stroke of the yoke where the velocity of the yoke is greatest. Consequently, since the bearing load factor is directly related to the product of pressure and velocity, it can be seen that the straight track design actually generates excessive bearing load since both factors are at a maximum at the same points during the stroke. Thus, a substantial side load is imposed on the yoke guide which causes increased friction and consequent heat buildup in the tool.
Accordingly, it is the primary object of the present invention to provide a scotch yoke design having a track configuration that minimizes the side load forces imposed on the yoke guides and thereby substantially eliminate the friction and heat generation normally associated with the yoke guides. In general, this is accomplished by fashioning the yoke with a curved track to eliminate the couple on the yoke shaft which results from the straight track configuration. In addition, as will subsequently be described in greater detail, the radius of curvature of the yoke track is selected in the peferred embodiment to be substantially equal to the distance between the crank pin and the center or the lower shaft bearing when the yoke is at its midstroke position so that the yoke guide load is perfectly balanced when the velocity of the reciprocating yoke is greatest.
Additional objects and advantages of the present invention will become apparent from a reading of the detailed description of the preferred embodiment which makes reference to the following set of drawings in which :